Method for Intelligent Load Management in Off-Grid AC Systems

ABSTRACT

The current disclosure provides methods and systems for intelligent load management in off-grid AC systems and provides methods and systems to control and prioritize loads, so that supply and demand can be balanced via an extremely robust and reliable system.

BACKGROUND OF THE INVENTION 1) Field of the Invention

The current disclosure provides methods for intelligent load managementin off-grid AC systems and provides methods to control and prioritizeloads, so that supply and demand can be balanced via an extremely robustand reliable system.

2) Description of Related Art

In an off-grid application typically available power is limited and thetotal load power exceeds the maximum power generation capability.Therefore, it becomes important to manage and prioritize loads based oncurrent system status. This is similar to demand response in gridsystems. Additionally, if energy storage is available, the battery stateof charge must be managed: the battery should act as a power source whenpower demand is high and recharge when extra power is available.

In a grid-connected application the frequency of the AC voltage istightly controlled by the power utility and any local inverter—forexample a solar inverter—has to be phase-synchronized to the grid.However, in an off-grid application there is no such constraint: thelocal inverter establishes the AC voltage and determines its frequency.

Accordingly, it is an object of the present invention to change the ACvoltage frequency to provide a channel of communication between theinverter and the rest of the system that can be used for loadmanagement. The converter varies the AC frequency as a function ofavailable power generation capacity, making that informationinstantaneously available throughout the system and intelligent loadsadjust their operation based on this signal. In particular, loads can beprioritized and system overloading can be avoided. This increases systemrobustness and ensures that more important loads are preferentiallyserved.

SUMMARY OF THE INVENTION

The above objectives are accomplished according to the presentdisclosure by providing in a first embodiment an off-grid power systemThe system may include at least one local power generator, at least onebattery, at least one variable frequency inverter for establishing ACvoltage and determining AC frequency, a channel of communication formedvia changing the AC voltage, at least one converter that varies ACfrequency as a function of available power generation capacity, whereinthe varying AC frequency comprises information that is availablethroughout the system, and at least one energy load, wherein the atleast one energy load is adjusted in response to the varying ACfrequency to prioritize the at least one energy load to avoid systemoverloading, wherein the off-grid system does not include a separatecommunication channel. Further, an inverter frequency may be controlledin the range of f_(min)<f_(nom)<f_(max), wherein f_(min) is the minimumfrequency, f_(nom) is the nominal frequency and f_(max) is the maximumfrequency. Still yet, a lower frequency indicates less power isavailable and total load is reduced. Further still, a higher frequencyindicates that extra power is available for the off-grid system. Stillagain, the system may include trip points for load disconnect. Furtheragain, the load disconnect may activate when the AC frequency fallsbelow a preset value to prioritize at least one load. Still yet, thesystem may include at least two loads wherein the at least two loads arelisted in order of decreasing priority, wherein the lowest priority loadwill be shut off when the AC frequency falls below a preset value.Further, the battery is in a charge mode when the AC frequency is abovea preset value and the battery supplies power to the at least one loadwhen the AC frequency is below the preset value. Still yet, the presetvalue may comprise a frequency range with a high frequency and a lowfrequency point defined to condition battery operation.

In a further embodiment, a method is provided for managing andprioritizing loads based on system status. The method may includeestablishing AC voltage and determining AC frequency via a localinverter, creating a channel of communication via changing AC voltagefrequency, varying AC frequency, via a converter, as a function ofavailable power generation capacity, wherein the varying AC frequencycomprises information that is available throughout the system, andadjusting at least one intelligent load based on the varying ACfrequency to prioritize the intelligent load and avoid systemoverloading, wherein the AC frequency controls source power in stepsfrom a negative value when frequency is low to full power delivered tothe at least one intelligent load when signaling frequency is high.Further, the method may not employ a separate communication channel butinstead the AC frequency signal communicates to an entirety of thesystem. Still yet, the method may comprise controlling an inverterfrequency in the range of f_(min)<f_(nom)<f_(max), wherein f_(min) isthe minimum frequency, f_(nom) is the nominal frequency and f_(max) isthe maximum frequency. Again, the method may include reducing total loadwhen a lower frequency indicates less power is available. Further again,the method may include a higher frequency indicating that extra power isavailable for the system. Still yet, the method employ trip points forload disconnect. Again, the method may include activating the loaddisconnect when the AC frequency falls below a preset value toprioritize at least one load. Yet further, the method may includelisting at least two loads in order of decreasing priority and shuttingoff a lowest priority load when the AC frequency falls below a presetvalue. Still further, the method may include placing a battery in acharge mode when the AC frequency is above a preset value and supplyingpower from the battery to the at least one intelligent load when the ACfrequency is below the preset value. Further yet, the method may includethe preset value comprising a frequency range with a high frequency anda low frequency point defined to condition battery operation.

BRIEF DESCRIPTION OF THE DRAWINGS

The construction designed to carry out the invention will hereinafter bedescribed, together with other features thereof. The invention will bemore readily understood from a reading of the following specificationand by reference to the accompanying drawings forming a part thereof,wherein an example of the invention is shown and wherein:

FIG. 1 shows an off-grid AC system.

FIG. 2 shows a load management strategy based on operating frequency ofthe current disclosure.

FIG. 3 shows a simulated system featuring a variable frequency ACvoltage source, five loads with a controllable disconnect, and a batteryinterfaced through a current-controlled AC/DC converter.

FIG. 4 shows Table 1, which shows values and frequencies for batterycontrol.

FIG. 5 shows the zero-crossing frequency detection algorithm outputmatches the actual AC frequency throughout the entire simulation.

FIG. 6 the voltage, current, and power from the AC source.

It will be understood by those skilled in the art that one or moreaspects of this invention can meet certain objectives, while one or moreother aspects can meet certain other objectives. Each objective may notapply equally, in all its respects, to every aspect of this invention.As such, the preceding objects can be viewed in the alternative withrespect to any one aspect of this invention. These and other objects andfeatures of the invention will become more fully apparent when thefollowing detailed description is read in conjunction with theaccompanying figures and examples. However, it is to be understood thatboth the foregoing summary of the invention and the following detaileddescription are of a preferred embodiment and not restrictive of theinvention or other alternate embodiments of the invention. Inparticular, while the invention is described herein with reference to anumber of specific embodiments, it will be appreciated that thedescription is illustrative of the invention and is not constructed aslimiting of the invention. Various modifications and applications mayoccur to those who are skilled in the art, without departing from thespirit and the scope of the invention, as described by the appendedclaims. Likewise, other objects, features, benefits and advantages ofthe present invention will be apparent from this summary and certainembodiments described below, and will be readily apparent to thoseskilled in the art. Such objects, features, benefits and advantages willbe apparent from the above in conjunction with the accompanyingexamples, data, figures and all reasonable inferences to be drawntherefrom, alone or with consideration of the references incorporatedherein.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

With reference to the drawings, the invention will now be described inmore detail. Unless defined otherwise, all technical and scientificterms used herein have the same meaning as commonly understood to one ofordinary skill in the art to which the presently disclosed subjectmatter belongs. Although any methods, devices, and materials similar orequivalent to those described herein can be used in the practice ortesting of the presently disclosed subject matter, representativemethods, devices, and materials are herein described.

Unless specifically stated, terms and phrases used in this document, andvariations thereof, unless otherwise expressly stated, should beconstrued as open ended as opposed to limiting. Likewise, a group ofitems linked with the conjunction “and” should not be read as requiringthat each and every one of those items be present in the grouping, butrather should be read as “and/or” unless expressly stated otherwise.Similarly, a group of items linked with the conjunction “or” should notbe read as requiring mutual exclusivity among that group, but rathershould also be read as “and/or” unless expressly stated otherwise.

Furthermore, although items, elements or components of the disclosuremay be described or claimed in the singular, the plural is contemplatedto be within the scope thereof unless limitation to the singular isexplicitly stated. The presence of broadening words and phrases such as“one or more,” “at least,” “but not limited to” or other like phrases insome instances shall not be read to mean that the narrower case isintended or required in instances where such broadening phrases may beabsent.

This disclosure applies to an off-grid system, such as an off-gridhouse, building or industrial factory. In an off-grid applicationtypically available power is limited and the total load power exceedsthe maximum power generation capability. Therefore, it becomes importantto manage and prioritize loads based on current system status. This issimilar to demand response in grid systems. Additionally, if energystorage is available, the battery state of charge must be managed: thebattery should act as a power source when power demand is high andrecharge when extra power is available.

In a grid-connected application the frequency of the AC voltage istightly controlled by the power utility and any local inverter—forexample a solar inverter—has to be phase-synchronized to the grid.However, in an off-grid application there is no such constraint: thelocal inverter establishes the AC voltage and determines its frequency.Changing the AC voltage frequency provides a channel of communicationbetween the inverter and the rest of the system that can be used forload management. According to the invention, the converter varies the ACfrequency as a function of available power generation capacity, makingthat information instantaneously available throughout the system.Intelligent loads adjust their operation based on this signal. Inparticular, loads can be prioritized and system overloading can beavoided. This increases system robustness and ensures that moreimportant loads are preferentially served.

Changing the AC voltage frequency provides a channel of communicationbetween the inverter and the rest of the system that can be used forload management. According to the invention, the converter varies the ACfrequency as a function of available power generation capacity, makingthat information instantaneously available throughout the system.Intelligent loads adjust their operation based on this signal. Inparticular loads can be prioritized and system overloading can beavoided. This increases system robustness and ensures that moreimportant loads are preferentially served.

In a preferred embodiment of this invention, the inverter frequency iscontrolled in the range:

f_(min)<f_(nom)<f_(max)  (1)

where f_(min) is the minimum frequency, f_(nom) is the nominal frequencyand f_(max) is the maximum frequency. A lower frequency indicates thatless power is available and total load should be reduced. A higherfrequency indicates that extra power is available. The off-grid systemof FIG. 1 includes local power generation, a variable-frequency inverterpowering intelligent loads and, optionally, a battery energy storagesystem. The AC frequency of the inverter is varied to indicate availablegeneration power. Loads can sense the AC frequency and reactaccordingly. A simple scheme has trip points, as known to those of skillin the art, for load disconnect when the frequency goes below a certainvalue. This provides a mechanism to prioritize loads. This is shown inFIG. 2. Loads 1 to n are listed in order of decreasing priority, so thatLoad n will be the first to turn off as AC frequency goes down. On theother hand, Load 1 will be on up to a lower AC frequency. The batterystorage is in charge mode when the frequency is high and extra power isavailable, it is off in an intermediate range and operates in dischargemode providing power to loads when the AC frequency is low, indicatingthat less generation power is available.

The AC frequency of the inverter is varied to indicate availablegeneration power. Loads can sense the AC frequency and reactaccordingly. A simple scheme has trip points for load disconnect whenthe frequency goes below a certain value, such as a present AC frequencyvalue. This provides a mechanism to prioritize loads. This is shown inFIG. 2. Loads 1 to n are listed in order of decreasing priority, so thatLoad n will be the first to turn off as AC frequency goes down. On theother hand, Load 1 will be on up to an even lower AC frequency. Thebattery storage is in charge mode when the frequency is high, i.e.,above a preset AC frequency, and extra power is available, it is off inan intermediate range, such as a preset range of frequencies with afirst and second present range points that determine/condition batteryoperation, and operates in discharge mode providing power to loads whenthe AC frequency is low, i.e., below a set frequency, indicating thatless generation power is available.

More complex schemes may be devised. Trip points may change based onother constraints (e.g., the HVAC trip point may change if the housetemperature becomes uncomfortable, making it a higher priority load).Certain variable loads may be varied continuously as a function offrequency. Different frequency ranges may be used to command differentsystem operating modes.

It is known that the grid utilizes frequency control as one of theancillary services required for reliable grid operation. However, thecurrent disclosure applies to a completely different type of systems:off-grid AC systems.

Advantages of the proposed method include: (1) the method providesreal-time intelligent management of loads, improving systemfunctionality and performance; (2) allows flexible prioritization ofloads; (3) does not require a separate communication channel, which mayfail; (4) is robust and has high reliability—if AC power is available,the information is automatically provided to the entire system.

This disclosure applies to an off-grid system, such as an off-gridhouse, building or industrial factory. In an off-grid applicationtypically available power is limited and the total load power exceedsthe maximum power generation capability. Therefore, it becomes importantto manage and prioritize loads based on current system status. This issimilar to demand response in grid systems. Additionally, if energystorage is available, the battery state of charge must be managed: thebattery should act as a power source when power demand is high andrecharge when extra power is available.

In a grid-connected applications, the frequency of the AC voltage istightly controlled by the power utility and any local inverter—forexample a solar inverter—has to be phase-synchronized to the grid.However, in an off-grid application there is no such constraint: thelocal inverter establishes the AC voltage and determines its frequency.Changing the AC voltage frequency provides a channel of communicationbetween the inverter and the rest of the system that can be used forload management. According to the invention, the converter varies the ACfrequency as a function of available power generation capacity, makingthat information instantaneously available throughout the system.Intelligent loads adjust their operation based on this signal. Inparticular loads can be prioritized and system overloading can beavoided. This increases system robustness and ensures that moreimportant loads are preferentially served.

More complex schemes could be devised. Trip points may change based onother constraints (the HVAC trip point may change if the housetemperature becomes uncomfortable, making it a higher priority load).Certain variable loads may be varied continuously as a function offrequency. Different frequency ranges may be used to command differentsystem operating modes.

It is well known that the grid utilizes frequency control as one of theancillary services required for reliable grid operation. Note that thepresent invention applies to a completely different type of systems:off-grid AC systems.

Advantages of the proposed method are: (1) the method provides real-timeintelligent management of loads, improving system functionality andperformance; (2) the method allows flexible prioritization of loads; and(3) the method does not require a separate communication channel, whichmay fail; and (4) the method is robust and has high reliability—if ACpower is available, the information is automatically provided to theentire system

For validation and proof of concept the system of FIG. 1 was simulatedin PLECS (Piecewise Linear Electrical Circuit Simulation) using thesimple load management strategy depicted in FIG. 2. The schematic of thesimulated system is shown in FIG. 3.

The system operated at a nominal frequency of 60 Hz, with variationallowed in the range 58<f<62 according to the limits prescribed in (1).To demonstrate the concept, the AC frequency was slowly ramped throughthe full range of frequencies, starting at 58 Hz and ending at 62 Hz. Inresponse to the changing frequency, several loads toggle from off to on,and the battery transitions from discharging to charging.

As shown in FIG. 2, each of the five loads simply connects ordisconnects at a frequency threshold, with lower thresholds for higherpriority loads. The battery is controlled with a frequency-modulatedscheme which takes on five discrete levels. The values and correspondingfrequencies are shown in Table I, see FIG. 4.

A frequency crossing detection algorithm based on detection of zerocrossings in the AC waveform is utilized as an example of how loadsmight sense and respond to the AC frequency. The real AC frequency andoutput of the detection algorithm are shown to match extremely well inFIG. 5.

As it ramps up, the frequency crosses turn-on thresholds for each of the5 loads, and it crosses the boundaries for the set-point current of thebattery as given in Table 1, see FIG. 4. These threshold changes areobserved in the current demanded from the AC supply, resulting instepped changes in power. FIG. 6 shows the AC voltage, AC current and ACpower supplied by the AC source. The AC voltage amplitude is always 120Vrms so that the AC voltage envelope is a constant band. When thecommand frequency is low, all loads are off and the battery providespower back to the AC source and AC power is negative. As the signalingfrequency increases, loads switch on at their threshold point andbattery current changes polarity as in Table I, see FIG. 4. The netresult is that the frequency signal controls the source power in stepsfrom a negative value when frequency is low to full power delivered tothe loads when signaling frequency is high. This is the desired behaviorwith power consumption being controlled by the signaling frequency.

All patents, patent applications, published applications, andpublications, databases, websites and other published materials referredto throughout the entire disclosure herein, unless noted otherwise, areincorporated herein by reference in their entirety.

While the present subject matter has been described in detail withrespect to specific exemplary embodiments and methods thereof, it willbe appreciated that those skilled in the art, upon attaining anunderstanding of the foregoing may readily produce alterations to,variations of, and equivalents to such embodiments. Accordingly, thescope of the present disclosure is by way of example rather than by wayof limitation, and the subject disclosure does not preclude inclusion ofsuch modifications, variations and/or additions to the present subjectmatter as would be readily apparent to one of ordinary skill in the artusing the teachings disclosed herein.

What is claimed is:
 1. An off-grid system comprising: at least one localpower generator; at least one battery; at least one variable frequencyinverter for establishing AC voltage and determining AC frequency; achannel of communication formed via changing the AC voltage; at leastone converter that varies AC frequency as a function of available powergeneration capacity, wherein the varying AC frequency comprisesinformation that is available throughout the system; at least one energyload, wherein the at least one energy load is adjusted in response tothe varying AC frequency to prioritize the at least one energy load toavoid system overloading; and wherein the off-grid system does notinclude a separate communication channel.
 2. The off-grid system ofclaim 1, wherein an inverter frequency is controlled in the range off_(min)<f_(nom)<f_(max), wherein f_(min) is the minimum frequency,f_(nom) is the nominal frequency and f_(max) is the maximum frequency.3. The off-grid system of claim 1, wherein a lower frequency indicatesless power is available and total load is reduced.
 4. The off-gridsystem of claim 1, wherein a higher frequency indicates that extra poweris available for the off-grid system.
 5. The off-grid system of claim 1,further comprising trip points for load disconnect.
 6. The off-gridsystem of claim 5, wherein the load disconnect activates when the ACfrequency falls below a preset value to prioritize at least one load. 7.The off-grid system of claim 1, comprising at least two loads whereinthe at least two loads are listed in order of decreasing priority,wherein a lowest priority load will be shut off when the AC frequencyfalls below a preset value.
 8. The off-grid system of claim 1, whereinthe at least one battery is in a charge mode when the AC frequency isabove a preset value and the at least one battery supplies power to theat least one load when the AC frequency is below the preset value. 9.The off-grid system of claim 8, wherein the preset value comprises afrequency range with a high frequency and a low frequency point definedto condition battery operation.
 10. A method for managing andprioritizing loads based on system status comprising: establishing ACvoltage and determining AC frequency via a local inverter; creating achannel of communication via changing AC voltage frequency; varying ACfrequency, via a converter, as a function of available power generationcapacity, wherein the varying AC frequency comprises information that isavailable throughout the system; and adjusting at least one intelligentload based on the varying AC frequency to prioritize the intelligentload and avoid system overloading, wherein the AC frequency controlssource power in steps from a negative value when frequency is low tofull power delivered to the at least one intelligent load when signalingfrequency is high.
 11. The method of claim 10, wherein the method doesnot employ a separate communication channel but instead the AC frequencycommunicates to an entirety of the system.
 12. The method of claim 10,further comprising controlling an inverter frequency in the range off_(min)<f_(nom)<f_(max), wherein f_(min) is the minimum frequency,f_(nom) is the nominal frequency and f_(max) is the maximum frequency.13. The method of claim 10, further comprising reducing total load whena lower frequency indicates less power is available.
 14. The method ofclaim 10, further comprising a higher frequency indicating that extrapower is available for the system.
 15. The method of claim 10, furthercomprising employing trip points for load disconnect.
 16. The method ofclaim 15, further comprising activating the load disconnect when the ACfrequency falls below a preset value to prioritize at least one load.17. The method of claim 10, further comprising listing at least twoloads in order of decreasing priority and shutting off a lowest priorityload when the AC frequency falls below a preset value.
 18. The method ofclaim 10, further comprising placing at least one battery in a chargemode when the AC frequency is above a preset value and supplying powerfrom the at least one battery to the at least one intelligent load whenthe AC frequency is below the preset value.
 19. The method of claim 18,further comprising the preset value comprising a frequency range with ahigh frequency and a low frequency point defined to condition batteryoperation.